The present invention concerns a closed kneader for kneading viscous materials such as plastic, rubber, etc., and more specifically the shape of the rotor shaft thereof.
An example of a conventional closed kneader is indicated in FIG. 4 and FIG. 5. In the drawings, a closed kneader 50 is composed of a kneading tank (hereinafter referred to as “chamber 3”) for holding kneaded material, a pair of rotors 51, 51, which are provided at both ends with rotor drive shafts 54a, 54b (hereinafter simply referred to as 54 when generically mentioned) passing through side walls 7a, 7b of the chamber 3, and which consist of a rotor shaft 52 forming a rotor blade 53 for kneading the material kneaded in the chamber 3, and a pressure cover 8. When kneading material, the kneaded material is poured into the chamber 3 by a proper means in a state where the pressure cover 8 is opened upward (see the position of 8a in FIG. 4), the pressure cover 8 is let down (see the position of the solid line in FIG. 4), and the rotors 51 are rotatebly driven with a driving means such as a motor, etc. connected to the rotor drive shafts 54.
One of the pair of rotor drive shafts 54 may be constructed as a driven shaft by using a connecting means such as gears, etc.
In kneading work using a conventional closed kneader, the proportion of the radius of the rotor shaft Rr′ against the radius on the inner face of the chamber Rc′ (Rr′/Rc′, also applicable hereinafter) is 0.5 or so, the proportion of the land width w′ of the rotor blade 53 against the radius of the rotor shaft Rr′ (w′/Rr′, also applicable hereinafter) is 0.3 or so, and the clearance m′ between the outer circumferential face of the rotor shaft and the wall face of the chamber is about 50% of the radius on the inner face of the chamber Rc′. This means a large clearance between the outer circumferential face of the rotor shaft and the inner wall face of the chamber, providing a thickness for the material to be mixed in the chamber.
During kneading, a lot of heat is produced within the kneaded material, in the chamber and also by the rotor shaft because of internal heat generation due to shearing and dispersion, etc. during kneading. For that reason, insufficient cooling occurs in the inner part of the kneaded material with respect to the kneading speed, i.e. the speed of heat generation, in the case of a kneaded material of large thickness and low thermal conductivity (especially rubber, etc.), even if cooling water is circulated through the chamber wall and the rotor shaft.
While sufficient cooling may be made in the kneaded material with little increase of internal temperature in a material with a small mixing volume, such a machine is inferior in productivity and therefore unrealistic as a mass production unit.
In the closed kneader, while a general kneading process is divided into a primary kneading for mixing without containing any vulcanizing agent line, and a secondary kneading for performing kneading by mixing the kneaded material which has been submitted to a certain kneading process in the primary kneading with a vulcanizing agent, the kneading material temperature must be kept no higher than a certain level (variable depending on the material) for mixing in a vulcanizing agent line.